The present invention relates to a circuit configuration having a semiconductor switch and a driver circuit, which has an input terminal for applying a switching signal and an output terminal. The output is connected to a control input of the semiconductor switch, for supplying a drive signal on the basis of the switching signal.
Such circuit configurations are used to switch currents, for example in switched-mode power supplies, bridge circuits or, generally, to connect a load connected in series with the load path of the semiconductor switch to a supply voltage. In this context, the circuit configurations are able to switch relatively high currents or voltages with short switching times. Electromagnetic radiation caused by the switching process and radiated by the circuit has more energy and a higher frequency, and thus causes more interference, the faster the semiconductor switch is driven, that is to say the switch changes over from the on to the off state or from the off to the on state.
A drive signal which is provided for driving the semiconductor switch and is supplied to the control input of the semiconductor switch assumes one of two drive levels, depending on whether the semiconductor switch is intended to be on or off. To limit the interference radiation, the known circuit configurations usually have driver circuits in which the gradient of the edges of the drive signals, and hence the gradient of the edges of the output signal from the semiconductor switch, are limited during the change from one drive level to the other. During the switching process, losses arise on the semiconductor switch which are particularly high if as when inductive loads are driven in bridge circuitsxe2x80x94high currents flow through the semiconductor switch during changeover. The electrical power which is then converted into heat on the semiconductor switch rises as the switching frequency increases, and can cause significant heating in the control circuit (usually in the form of an integrated circuit), which, in the worst case, results in components being destroyed.
The object of the invention is to provide a circuit for driving a semiconductor switch which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind. Provision should be made for the gradient of the drive signal to be set on the basis of temperature during the changeover from one drive level to the other. The gradient of the drive signal increases, in particular, with rising temperature.
With the above and other objects in view there is provided, in accordance with the invention, a circuit configuration, comprising:
a semiconductor switch having a control input;
a driver circuit having an input terminal for receiving a switching signal and an output terminal connected to the control input of the semiconductor switch;
the output terminal carrying a drive signal based on the switching signal, the drive signal respectively assuming a first drive level and a second drive level, wherein a gradient of the drive signal during a change from the first drive level to the second drive level is temperature-dependent.
If the temperature of the circuit is in a temperature range in which, as a result of the semiconductor switch switching slowly, an additional temperature rise can occur without the risk of destroying the circuit, the gradient of the drive signal is reduced in order to reduce the electromagnetic interference radiation. If the temperature in the circuit configuration rises and there is concern regarding the risk of components being destroyed in the event of a further temperature rise, the gradient of the drive signal increases during a change from one drive level to the other in order to achieve fast switching in the semiconductor switch and, thereby, to limit the switching losses which result in further heating. To protect the components, the invention accepts more intense electromagnetic radiation in order to prevent that the components overheat.
In accordance with an added feature of the invention, the gradient of the drive signal increases with rising temperature.
In accordance with an additional feature of the invention, the driver circuit includes a current source for supplying a temperature-dependent current.
In accordance with a preferred embodiment of the invention, the current source can be selectively connected to the output terminal based on the switching signal.
In other words, the driver circuit has a current source and a current sink for supplying and, respectively, for receiving a temperature-dependent current. The current source and the current sink can thereby be connected to the output terminal on the basis of the switching signal. This embodiment is of particular use when a field-effect transistor is used as the semiconductor switch. The gate electrode, as the control connection of the semiconductor switch, is then connected to the output terminal of the driver circuit. The conductivity of the field-effect transistor is dependent on the charge stored in its gate capacitance and/or on the voltage between its gate electrode and source electrode. If the field-effect transistor is intended to be on, a current flows through the output terminal to its gate capacitance, which causes the gate/source voltage, as the drive signal critical for the field-effect transistor, to rise. The speed of the rise in or the gradient of the drive signal is approximately proportional to the capacitance value of the gate capacitance and to the current flowing to the gate capacitance. If this current rises with rising temperature, the field-effect transistor is turned on more quickly, which reduces the losses during the changeover phase from the off to the on state. If the field-effect transistor is intended to be off, its gate capacitance is discharged with a temperature-dependent current predetermined by the current sink, in which case the discharge, and hence the changeover from the on to the off state, takes place more quickly the higher the current.
In accordance with a further feature of the invention, the driver circuit includes a current source for outputting a temperature-dependent current to the output terminal and a current sink for receiving a temperature-dependent current from the output terminal.
In accordance with again an added feature of the invention, the driver circuit has a first switch for connecting the current source to the output terminal and a second switch for connecting the current sink to the output terminal based on the switching signal.
In accordance with again an additional feature of the invention, the current rises with rising temperature. In a specific feature of the invention, the current rises with rising temperature after a predetermined temperature threshold has been reached.
In accordance with again another feature of the invention, the driver circuit includes a drive unit connected to and driving the first switch and the second switch based on the switching signal, wherein the drive unit has an input receiving the switching signal.
In accordance with still another feature of the invention, there is provided a temperature sensor connected to the current source and to the current sink for setting the current.
In accordance with still a further feature of the invention, the semiconductor switch is a power MOSFET.
In accordance with a concomitant feature of the invention, the first switch and the second switch are each transistors each having a gate connected to and receiving from the drive unit a signal dependent on the switching signal.
The current source and the current sink are in the form of controllable current sources to which a temperature sensor supplies a temperature signal for setting the current.
The driver circuit preferably has only one controlled current source, which can be connected to the output terminal via a first current mirror, as the current source, and via a second current mirror, as the current sink.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for driving a semiconductor switch, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.